1. Field of the Invention
The present invention relates in general to a low voltage, low current, supply switching circuit, and in particular to a method and apparatus for selectively maintaining circuit power when higher supply voltages are present.
2. Description of the Related Art
Almost all modern electronics utilize memory or other circuits that require power even after system power is turned off. Such power is typically called backup power, and is used to maintain timekeeping and Random Access Memory (RAM) for such purposes as program storage and other data storage that may be needed when main power is restored. Backup power can also be used as a bridge for situations when main power is disabled for a short period of time to enable users to turn off devices in an orderly manner.
Typically, a battery or a small amount of current from the main supply that feeds a capacitor is used to provide this backup power. In mobile devices, where the device is not plugged into an electrical outlet, battery power is almost always used. As such, backup power requirements must include circuits that attempt to maximize the life of the battery used for backup power. Typically, a circuit is provided that switches the battery off, i.e., such that the battery is disconnected from the load, when the higher voltage main power supply is applied to the circuit.
As mobile devices, e.g., cellular telephones, Personal Data Assistants (PDAs), laptop computers, etc. are decreased in size and weight, smaller batteries are used, and therefore, such devices must also decrease power consumption. Further, such devices have added new features, e.g., call waiting in cellular telephones, internet access from PDAs, Global Positioning System (GPS) receivers used for location services in cellular telephones, laptop computers, and PDAs, etc. As such, the main circuitry, as well as circuitry that may need backup power, has increased, and, along with it, power consumption.
Typically, to decrease power consumption, supply voltages of the device are decreased. However, standard batteries are currently only available in pre-defined voltages (e.g. 1.5V, 3.0V). Therefore, it is possible that a desired decreased supply voltage falls in between two standard battery voltages, where only the higher voltage battery is usable in the design. This instance presents a problem for typical switching solutions, because the typical solution for switching the power may not turn the backup battery off when main power is applied.
It can be seen, then, that there is a need in the art for a method and apparatus that can provide backup power to electronic circuits on demand. It can also be seen that there is a need in the art for a method and apparatus that turns off the backup power supply whenever main power is on regardless of main power voltage. It can also be seen that there is a need in the art for a method and apparatus that can turn the backup power supply off even with additional circuitry attached to the backup circuits. It can also be seen that there is a need in the art for a method and apparatus for extending backup battery life in a device needing battery backup by automatically disconnecting the backup battery when the lower voltage main power source is activated.